1. Field of the Invention
This invention relates to processing of image signals for detail improvement and noise reduction and, more specifically, to detail processing apparatus and method that operate separately upon horizontal and vertical detail.
2. Description Relative to the Prior Art
It is sometimes desirable to operate separately upon vertical and horizontal detail in an image. For example, it is easier to design separable processing into a real time, pipelined digital architecture. Sometimes, it may be advantageous to separately modify the vertical and horizontal detail signals by a non-linear process, e.g., by coring the digital signals, before combining the modified detail with the image signals.
In U.S. Pat. No. 4,609,938, a digital luminance processing circuit generates vertical and horizontal contour signals that are separately modified and added to a luminance signal. The vertical contour circuit includes a series arrangement of a vertical high pass filter and a low pass filter. The vertical high pass filter includes delay lines for processing three adJacent lines in order to filter out a component having a large vertical variation (on a TV screen). The low pass filter operates on the vertical contour signal to eliminate the chrominance subcarrier component by calculating a linear combination of image samples weighted so as to cancel the chrominance subcarrier.
Separating vertical and horizontal detail processing works well with detail that has either a predominant vertical or horizontal orientation. One or the other detail circuit will detect the detail and provide a corresponding detail output. Diagonally-oriented detail, however, gets doubly-enhanced in such a system because diagonal detail has both vertical and horizontal components. Each are separately detected and both circuits provide corresponding output, thus--when combined--doubling the boost for diagonal detail relative to vertical or horizontal detail and thereby creating undesirable artifacts in the image.
It is desirable to use a "detail out of green" approach for edge enhancement, since there are ordinarily more green samples to work with and the green record has less noise and artifacts. The green record also corresponds to the peak visual response, so that the eye is more receptive to detail in green. To implement such an approach, the green channel is tapped for detail at an appropriate point, the detail is processed, and the processed detail is re inserted into the channel. In cameras with sophisticated signal processing, a color correction matrix corrects the spectral sensitivities of the image sensor for the chromaticities of the display. However matrixing the green signal changes the green signal in relation to the sensor signal level and increases the noise in the green channel, depending on the magnitude of the "red into green" and "blue into green" matrix terms. This means that using the matrixed green signal as input for signal enhancement tends to enhance the noise as well as the detail. The contrary approach, that of enhancing the green signal prior to color matrixing, also has drawbacks. The matrix is intended to remove unwanted color sensitivities, i.e., color crosstalk, which are not scene-dependent. Adding a scene dependent gain adJustment (such as a detail boost) affects this complex interrelationship in a way the color matrix is not designed to handle--in effect adding a scene-dependent component that cannot be fully removed by the matrix.
An article by R. H. McMann, Jr. and A. A. Goldberg ("Improved Signal Processing Techniques for Color Television Broadcasting", Journal of the SMPTE, March 1968, pp. 221-228) represents a partial solution to this problem (in an analog processing system). In the McMann and Goldberg article, a "contour out of green" image enhancement approach is disclosed in which an image enhancer is installed in the green channel between the camera and an electronics unit. The detail signal from the image enhancer is fed into the R, G, B output channels of a masking amplifier, which provides a first order cross matrix correction for colorimetric errors due to the difficulty of exactly realizing the three color channel spectral response curves called for by theory. The green channel is tapped for detail prior to matrixing, that is, prior to increasing noise in the green channel; then the processed detail is added back after matrixing, thus not inserting a scene-dependent variable into the matrix. In a live camera, the masking amplifier is inserted in the signal path ahead of gamma conversion and performs a masking function in linear space. In a film camera, the masking amplifier is inserted after gamma correction and performs a masking function in gamma space, since this location provides a certain amount of multiplicative coupling to help compensate for the non-linear cross coupling of the film dyes. In both cases, the detail is extracted from the green channel prior to masking, i.e., either in linear or gamma space as dictated by the masking function. Then the processed detail is added back in the same space.
The problem with the McMann and Goldberg approach lies in the different requirements of edge enhancement relative to color correction. Color correction matrixing is an additive correction process that should be done in linear space for best effect. Edge enhancement, on the other hand, should provide a subJective sharpness improvement equally perceptible in black portions as in white portions of the displayed picture. In terms of the linear video signal provided by the sensor, this is a non-linear multiplicative operation on the signal, which is difficult to accomplish. Therefore, linear space edge enhancement excessively boosts detail in the dense parts of the image. In combination with the problem noted with diagonal detail, present detail processing techniques that must operate in conjunction with a colorimetric correction process tend to introduce artifacts in the image, either due to detail orientation or to density variations in the image detail.